Polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost. In particular, polyethylene (PE) is the one of the largest volume polymers consumed in the world. It is a versatile polymer that offers high performance relative to other polymers and alternative materials such as glass, metal, or paper. Polymer compositions such as, for example, PE compositions are used for the production of a wide variety of articles ranging from plastic films to drums and tanks. Plastic films such as PE films are mostly used in packaging applications, but they also find utility in the agricultural, medical, and engineering fields. PE films are manufactured in a variety of grades that are usually differentiated by the polymer density, for example, low density polyethylene (LDPE), medium density polyethylene (MDPE), and high density polyethylene (HDPE), wherein each density range has a combination of properties making it suitable for a particular application.
The melt index (MI) of a polymer may provide a simple indicator of resin processability and end-use performance. The processability and end-use performance are factors to be considered in the polymer's suitability for use in a particular application. Particularly, the MI is related to the ease of extrusion of the resin during fabrication and is further an indirect comparative measurement of the head pressure generated by the melt extrusion of the polymer in an extruder. For example, higher MI polymers are typically amenable to plastic shaping processes such as injection molding whereas lower MI polymers are typically amenable to plastic shaping processes such as blow molding or extrusion. Consequently, it would be desirable to modify the melt index of a polymer and likewise polymeric compositions comprising the polymer, thereby adjusting its suitability for a particular application.
Some common variables for controlling melt index are through adjustments of the reactor temperature, modification of the ethylene concentration, and/or the inclusion of hydrogen in the polymerization process. There are drawbacks associated with the use of the aforementioned variables for controlling melt index. For example, raising the reactor temperature will, at some point, cause fouling while lowering the ethylene concentration can result in a lower catalyst productivity. Further the drawbacks associated with the addition of hydrogen can be twofold in that the addition of hydrogen both increases the saturation pressure in the reactor which is typically compensated for by the addition of less ethylene monomer and may cause poor circulation and fouling within the reactor. Thus, there is a need to develop a method for modifying the MI that overcomes these shortcomings.